My invention relates to a system for sensing the speed of an electric motor such as a voice coil motor, and more specifically to a motor speed sensing system suitable for use in the feedback speed control of a transducer or transducers traveling from track to track in data storage and retrieval systems employing "fixed" or hard magnetic disks as record media, among other applications.
The voice coil motor represents one of the familiar means for positioning a transducer or electromagnetic head with respect to a multiplicity of annular, concentric data storage tracks on a hard magnetic disk. U.S. Pat. No. 4,807,072 to Ono et al., assigned to the assignee of the instant application, discloses a known example of transducer positioning system employing a voice coil motor.
In the above and other transducer positioning systems, the precise control of the traveling speed of the transducer is essential for the minimization of seek time, that is, the time required for transducer travel from a departure track to a destination track. U.S. Pat. No. 4,875,116 to Yasuda et al., also assigned to the assignee of the instant application, teaches the use of an optical encoder for sensing the speed of the voice coil motor. Optical encoders are, however, very expensive. A far less expensive motor speed sensor has therefore been generally employed, that is, that comprising a pair of permanent magnets and a speed sensing coil disposed opposite the magnets for reciprocating movement across their fields.
I object to the electromagnetic speed sensor as heretofore used for transducer position control purposes. The sensor must of necessity be disposed in the immediate proximity of the voice coil motor. This placement of the sensor has made it unavoidable for the speed sensing coil to generate an undesired voltage component for its mutual induction with the motor drive coil, in addition to the desired voltage proportional to the traveling speed of the transducer.
As far as I am aware, no truly satisfactory solution has yet been proposed for the elimination of the noise component from the electromagnetic speed sensor output. Any noise suppression means to be employed to this end must be capable of fully performing the function for which it is intended, but should not be so expensive that the total cost of the electromagnetic speed sensor and the noise suppression means will exceed, or even come close to, the cost of an optical encoder which might be adopted in place of the electromagnetic sensor without noise suppression means.